Performance optimization is making a program run faster, and also is closely related to refactoring
Typically, an 80/20 rule applies. A 20% effort is needed to implement the bulk of the program. Then another 80% effort (and budget) is needed to optimize, refactor, extend and/or document the program.
You first need the program to produce correct results (with correct input) before you know what is making it slow. Do not worry about performance during development.